He's about 4". I'm going to see about getting a 5 gallon tank for these cases. I already have the spare heater and filter. This tank also has an albino CAE, 5 Tiger Barbs, 3 long tail zebra danios. 1 clown loach and 2 Julli Cory. Tank as purchased with all fish included. For 1st week, 10% of water was exchanged daily for 1 week, then I started 25% weekly changes. This is 3rd week with this tank. All other fish seem to be fine. As a matter of fact, al are much better than they were prior to arring here.How big is he? You could put him in a clean 5g bucket for quarentine and dose Maracyn if you don't want to treat the rest of the fish. (you may need a spare filter/heater) Looks like fungus to me.
Also...
what are water params? Other tank mates? tank size & maitenence schedule?
he looks as he is suffering from bad water quality, could be a result of cycling the tank. I would up the water changes and make sure you make a daily note of the water parameters, keeping the ammonia and nitrites as close to zero as physically possible.
That's what I was thinking. The CAE might be sucking on his slime coat.I may be wrong, but it doesn't look like any type of disease to me. The first things that came to me immediately, were fight wounds or excessive flicking/flashing.
It doesn't seem as if you have any fish that would clash with the rainbow shark. Is it possible that there isn't proper hiding places for you bottom fish and they are squabbling over places to hid? If not then I would think flashing. The marks look the same as when my rainbow shark was in poor water quality years ago and was flashing excessively. I think good clean water is all that is really needed as Dangerdoll said.
What is "flashing"?
What would it look like on a red-tailed shark?
Any pix?